fix(deposits): FD Single Rate Calculator sent tenureInMonths=0 for tenures under 30 days

SYMPTOM
FD Single account create screen, Rate Calculator: entering a tenure
under 30 days (e.g. 27) and clicking "Find Rate" produced a Bad
Request - "The parameter `tenureInMonths` must be greater than 0."

ROOT CAUSE
calculateRate() (fixed-deposit-account-interest-rate-chart-step.
component.ts) always converted the Days-unit tenure to months before
calling the backend calculator:
  tenureInMonths = Math.floor(enteredDays / 30)
For any tenure under 30 days this floors to 0, which the backend
correctly rejects (paired backend commit: "FD Single Rate Calculator
crashed for tenures under 30 days").

FIX
- fixed-deposits.service.ts: calculateFixedDepositInterest() -
  tenureInMonths is now optional, added optional tenureInDays.
- fixed-deposit-account-interest-rate-chart-step.component.ts -
  calculateRate(): for a Days-unit tenure >= 30 days, unchanged
  (tenureInMonths = floor(days/30)); for < 30 days, sends tenureInDays
  instead and leaves tenureInMonths undefined. Also fixed the
  API-failure fallback's periodic-compounding math, which divided by
  tenureInMonths directly and would have produced NaN once
  tenureInMonths could legitimately be undefined - it now falls back
  to (enteredDays / 30) when tenureInMonths is absent.

SCOPE
Only the Rate Calculator preview inside the FD Single/Double/Special
account create screen. Does not touch how depositPeriod /
depositPeriodFrequencyId get sent on actual account submission.

IF THIS BREAKS SOMETHING LATER
- The same 400 reappearing for a short FD Single tenure -> confirm the
  Find Rate network call carries tenureInDays and NOT a zero
  tenureInMonths; check calculateRate()'s branch choosing between the
  two.
- A wrong maturity preview number specifically when the backend API
  call fails and the frontend falls back to its own formula -> check
  the `effectiveMonths = tenureInMonths ?? enteredDays / 30` fallback
  line in calculateRate().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/deposits/fixed-deposits/fixed-deposits.service.ts

@@ -133,7 +133,8 @@ export class FixedDepositsService {
   calculateFixedDepositInterest(params: {
     principalAmount: number;
     annualInterestRate: number;
-    tenureInMonths: number;
+    tenureInMonths?: number;
+    tenureInDays?: number; // alternative to tenureInMonths, for sub-month FD Single tenures
     interestCompoundingPeriodInMonths: number;
     interestPostingPeriodInMonths: number;
     submittedOnDate?: string; // yyyy-MM-dd — enables exact calendar calculation